1. Field of the Invention
The present invention relates to a driving control circuit for an ultrasonic motor that drives a rotor using traveling vibration waves generated in an elastic member by a piezoelectric member.
2. Related Background Art
A traveling vibration wave type ultrasonic motor is a motor that applies a frequency voltage to a piezoelectric member to cause it to flex and vibrate, thus causing the elastic member, to which the piezoelectric member is stuck, to generate traveling vibration waves, and pressing the rotor into contact with this elastic member to drive it by friction, as disclosed in Japanese Patent Laid-Open No. 59-111609.
This ultrasonic motor has such characteristics that if a load increases sharply while it is rotating, it diverts from resonance and stops. When it stops under this condition, the phase difference between a frequency voltage applied to the piezoelectric member (hereinafter called input voltage) and a monitor voltage (voltage generated by the deformation of a piezoelectric member) diverts from the value at the time of normal rotation, and the vibrations of the stator almost disappear. In this specification, such a state is called a vibration diversion state.
In a piezoelectric member having such characteristics, if variations in load occur due to changes in temperature and humidity, variations in pressure, or changes in viscosity of the lubricating oil in the bearings, the member is liable to enter the vibration diversion state, with the result that there is a possibility that the motor may suddenly stop, so its reliability of operation is low.
As a measure to counter such a vibration diversion state, a control device has been proposed that shifts the driving frequency to a restart frequency higher than the resonance frequency, after which it resets to a normal driving point by lowering the frequency, as disclosed in the specification of U.S. Ser. No. 152,644 dated Feb. 5, 1988, abandoned in favor of U.S. Ser. No. 361,744 filed May 30, 1989, now U.S. Pat. No. 4,914,337.
However, it is time-consuming for a voltage, the frequency of which is higher than the resonance frequency, to be applied to a piezoelectric member of such an ultrasonic motor, and then for a lower frequency voltage to be applied. For a motor that drives a lens-barrel in the focusing device of a camera, one that performs a recovery operation from the vibration diversion state more rapidly has been desired.
In addition, such an ultrasonic motor is a friction contact type motor, and has the problem that its starting characteristics are unstable. FIG. 7 is a figure illustrating this phenomenon. Input voltage V.sub.R is taken as the horizontal axis, and the number of rotations N is taken as the vertical axis.
When the input voltage V.sub.R is applied to a stopped ultrasonic motor by increasing the voltage gradually from zero, there are cases where the motor starts rotating slowly at voltage VR1, it starts at V.sup.R2 greater than voltage V.sup.R1 initially, and after starting, it rotates at a certain speed. Such phenomena are caused by differences in environment such as temperature, or in number of past uses, or in different manufacturing lots, among other things. So, by regarding this phenomenon at the starting time as one kind of vibration diversion state, starting by the above-described method can be considered. However, since the driving frequency is raised to a region higher than the resonance frequency and is then lowered to a normal frequency, a time lag occurs at the starting time.